1. Field of the Invention
This invention relates to an improved silver halide photographic material and particularly to a silver halide photographic lithographic light-sensitive material having a high contrast halftone gradation.
2. Description of the Prior Art
In photographic printing plate formation in the printing industry, a continuous tone photographic image is converted into a halftone screened image, which is then utilized to prepare a printing plate. Ordinarily three color separated halftone positives are produced from a photographic multi-color print by first making color separation negatives therefrom, exposing a lithographic light-sensitive material which is in an intimate contact with a contact screen or cross-hatched screen to light through each of these separation negatives, and then processing the exposed lithographic light-sensitive material with a lithographic developer. Recently, direct screening methods are becoming prevalent in photo plate making whereby a color separated halftone image can be obtained by exposure through a halftone screen and a color filter. While a conventional, indirect process gives halftone positives, such a direct process provides halftone negatives.
Such a screened image (negative or positive) comprises dots with different sizes, and the image density varies depending on the ratio of the area occupied by the dots. A greater change of the total dot area ratio implies a higher contrast on a halftone gradation. One can calculate the halftone contrast from the relationship between the logarithm of the exposure and the dot area ratio or the integrated density (e.g., as disclosed in J.A.C. Yule, Principles of Color Reproduction, p. 90-92, John Wiley & Sons Inc. (1967)).
The halftone contrast is quite important to determine the tonal rendition of the final print. Usually the contrast of a halftone negative should be higher than that of a halftone positive and particularly the contrast at the density range higher than the 50% dot area should be sufficiently high. This requirement is due to the fact that, in printed materials, the highlight rendition in which it is somewhat difficult to faithfully reproduce the extremely small dots during the printing process, is of great value; therefore, the contrast at the highlight area, i.e., at the high density region in the halftone negative, must be emphasized.
A usual technique in the printing industry adopted to increase the contrast of a halftone image is the use of a high contrast contact screen or of an auxiliary exposure generally called a no screen or highlight exposure. The former method, however, requires that various contact screens be maintained, requiring additional expense as well as complicated management of the screens, while the latter method has the difficulty of determination of exposure conditions and also a tendency for the dot quality to deteriorate. A vigorous stirring of the developer during processing of a photographic lithographic light-sensitive material can increase the halftone contrast, which method, however, cannot be employed in development in an automated processor.
Further, the dot quality as well as the halftone contrast of the image obtained by development of a photographic lithographic light-sensitive material also determines the final quality of the printing plate. A halftone screened image must comprise dots, each without fringe therearound and with a sufficiently high contrast. To achieve this requirement, processing with a hydroquinone type, a so called lithographic developer which contains a relatively low concentration of sulfite ion is conventionally employed. It is further well known in the art that incorporation of a polyethylene oxide in the photographic material is effective to improve the dot quality for such development.
To achieve a superior dot quality is an essential requirement for photographic lithographic light-sensitive materials.